Dynamic topological domain walls driven by lithium intercalation in graphene.

Stacking engineering in van der Waals (vdW) materials is a powerful method to control topological electronic phases for quantum device applications. Atomic intercalation into the vdW material can modulate the stacking structure at the atomic scale without a highly technical protocol. Here we report that lithium intercalation in a topologically structured graphene/buffer system on SiC(0001) drives dynamic topological domain wall (TDW) motions associated with stacking order change by using an in situ aberration-corrected low-energy electron microscope in combination with theoretical modelling.

Two-Dimensional Superconductivity of Ca-Intercalated Graphene on SiC: Vital Role of the Interface between Monolayer Graphene and the Substrate.

Ca-intercalation has enabled superconductivity in graphene on SiC. However, the atomic and electronic structures that are critical for superconductivity are still under discussion. We find an essential role of the interface between monolayer graphene and the SiC substrate for superconductivity.

Electron-Hole Recombination Time at TiO2 Single-Crystal Surfaces: Influence of Surface Band Bending.

Photocatalytic activity is determined by the transport property of photoexcited carriers from the interior to the surface of photocatalysts. Because the carrier dynamics is influenced by a space charge layer (SCL) in the subsurface region, an understanding of the effect of the potential barrier of the SCL on the carrier behavior is essential. Here we have investigated the relaxation time of the photoexcited carriers on single-crystal anatase and rutile TiO2 surfaces by time-resolved photoelectron spectroscopy and found that carrier recombination, taking a nanosecond time scale at room temperature, is strongly influenced by the barrier height of the SCL.

Metallic transport in a monatomic layer of in on a silicon surface.

We have succeeded in detecting metallic transport in a monatomic layer of In on an Si(111) surface, Si(111)-sqrt[7]×sqrt[3]-In surface reconstruction, using the micro-four-point probe method. The In layer exhibited conductivity higher than the minimum metallic conductivity (the Ioffe-Regel criterion) and kept the metallic temperature dependence of resistivity down to 10 K. This is the first example of a monatomic layer, with the exception of graphene, showing metallic transport without carrier localization at cryogenic temperatures.

Electron-phonon interaction and localization of surface-state carriers in a metallic monolayer.

Temperature-dependent electron transport in a metallic surface superstructure, Si(111)sqrt[3] x sqrt[3]-Ag, was studied by a micro-four-point probe method and photoemission spectroscopy. The surface-state conductivity exhibits a sharp transition from metallic conduction to strong localization at approximately 150 K. The metallic regime is due to electron-phonon interaction while the localization seemingly originates from coherency of electron waves.

Variable-temperature independently driven four-tip scanning tunneling microscope.

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively.

Four-point probe resistance measurements using PtIr-coated carbon nanotube tips.

We performed four-terminal conductivity measurements on a CoSi2 nanowire (NW) at room temperature by using PtIr-coated carbon nanotube (CNT) tips in a four-tip scanning tunneling microscope. The physical stability and high aspect ratio of the CNT tips made it possible to reduce the probe spacing down to ca. 30 nm.

Interaction between adatom-induced localized states and a quasi-two-dimensional electron gas.

Using angle-resolved photoemission spectroscopy, we investigate changes in the band dispersion of a free-electron-like surface state of [FORMULA: SEE TEXT], induced by adsorption of submonolayer Au adatoms. At room temperature, where the adatoms are in a two-dimensional adatom-gas phase, electrons are transferred from the Au adatoms to the substrate, shifting the surface band downwards and causing it to deviate from a parabolic dispersion. At 135 K where the Au adatoms are frozen at specific sites of the substrate, the band splits into two.

Electrical resistance of a monatomic step on a crystal surface.

We have succeeded in measuring the resistance across a single atomic step through a monatomic-layer metal on a crystal surface, Si(111)(sqrt[3]xsqrt[3])-Ag, using three independent methods, which yielded consistent values of the resistance. Two of the methods were direct measurements with monolithic microscopic four-point probes and four-tip scanning tunneling microscope probes. The third method was the analysis of electron standing waves near step edges, combined with the Landauer formula for 2D conductors.

Anisotropy in conductance of a quasi-one-dimensional metallic surface state measured by a square micro-four-point probe method.

We have devised a "square micro-four-point probe method" using an independently driven ultrahigh-vacuum four-tip scanning tunneling microscope, and succeeded for the first time to directly measure anisotropic electrical conductance of a single-atomic layer on a solid surface. A quasi-one-dimensional metal of a single-domain Si(111)4 x 1-In had a surface-state conductance along the metallic atom chains (sigma(axially)) to be 7.2(+/-0.